It is possible to observe a sample surface structure by scanning an electron beam and irradiating the sample with the electron beam and detecting secondary charged particles released from the sample at that time. An example of such electron beam devices is a scanning electron microscope (hereinafter, also referred to as the SEM).
Meanwhile, it is also possible to observe a sample surface structure by scanning an ion beam instead of the electron beam and irradiating the sample with the ion beam and detecting secondary charged particles released from the sample at that time. An example of such ion beam devices is a scanning ion microscope (hereinafter, also abbreviated as the SIM). In particular, when the sample is irradiated with the ion beam using ion species of a light mass, such as hydrogen and helium, in the ion beam device such as the scanning ion microscope, sputtering action relatively decreases, which is preferable to observe the sample.
A gas field ionization ion source is preferably used as an ion source of such an ion beam device. The gas field ionization ion source is the ion source that ionizes a gas using an electric field generated by an emitter tip and generates an ion beam. The gas field ionization ion source is configured to include a gas ionization chamber containing the emitter tip which has a needle shape and to which a high voltage can be applied, and an ionization gas (ion material gas) is supplied to the gas ionization chamber from the gas source via a gas supply piping.
In the gas field ionization ion source, when the ionization gases (or gas molecules) supplied from the gas supply piping approaches a distal end of the needle-shaped emitter tip to which the high voltage is applied and an intense electric field is applied, electrons inside the gases (gas molecules) tunnel through a potential barrier, which has been reduced by the intense electric field, due to a quantum tunneling effect, and the gases (gas molecules) are released as positive ions. These released ions are used as the ion beam in the ion beam device.
The gas field ionization ion source can generate an ion beam having a narrow energy width. In addition, a size of the ion generation source is small, and thus, it is possible to generate a fine ion beam.
Meanwhile, it is necessary to obtain an ion beam, with a high current density on a sample in order to observe the sample at a high signal to noise ratio (S/N ratio) in the ion beam device including the scanning ion microscope. In order for this, it is necessary to increase an ion radiation angle current density of the gas field ionization ion source. A molecular density of the ionization gas in the vicinity of the emitter tip may be increased in order to increase the ion radiation angle current density.
In this case, a gas molecular density per unit pressure is inversely proportional to temperature of the gas. In this regard, it is desirable to cool the emitter tip to cryogenic temperature and decrease the temperature of the ionization gas in the vicinity of the emitter tip. Accordingly, it is possible to increase the molecular density of the ionization gas in the vicinity of the emitter tip by cooling the emitter tip to the cryogenic temperature.
On the other hand, it is necessary to prevent vibration of a freezer, which is an ion beam device cooling mechanism that cools the emitter tip to the cryogenic temperature, from being transmitted to the emitter tip in order to observe the sample with high resolution in the ion beam, device including the scanning ion microscope. Thus, PTL 1 discloses an ion beam device cooling mechanism provided with a function of preventing transmission of vibration caused by a refrigerator to an emitter tip of a gas field ionization ion source, the ion beam device cooling mechanism in which the mechanical refrigerator and a helium gas pot are combined. A helium gas (inert gas) is stored in the helium gas pot as a cooling medium gas for cooling of the gas field ionization ion source.